As one of methods for evaluating slope safety, there is a method of calculating a factor of safety using a slope stability analysis formula, such as the Fellenius method and the modified Fellenius method. The factor of safety is an index for evaluating slope safety, and is represented by a ratio between slipping force which causes something to slips and resistance force which prevents the slippage. If this value becomes less than 1, that is, if the slipping force becomes larger than the resistance force, it is evaluated that collapse may occur.
The Fellenius method and the modified Fellenius method are methods of calculating a factor of safety, which is a ratio between slipping force that is based on gravity, and resistance force that is based on frictional force, adhesive force (cohesion), and the like, using clod weight, pore water pressure, clod cohesion, and an internal friction angle.
For example, there are methods described in PTL 1 to PTL 4, in relation to a technique for monitoring slope safety.